Over the years, the concepts of designs for the total knee arthroplasty have evolved to the point where with few exceptions, most are quite comparable in the design of femoral, tibial and patellar prostheses.
Major discrepancies and problems encountered are caused by physician error and failure to understand the principles of more complex alignment or ligament problems to be corrected at surgery. With the more complex alignment or "routine" degenerative knee, the major differences are the ease and consistency of instrumentation for alignment and proper bone cuts allowing proper ligament balance. This allows satisfactory motion and stability post operatively.
The distal femoral cuts must be placed to provide the knee prosthesis with a proper flexion and extension gap, proper varus-valgus alignment, proper patellofemoral relationship and proper rotation. It is customary to use an intramedullary rod placed in a retrograde fashion between the medial and lateral femoral condyles just anterior to the intercondylar notch to establish a single point of reference for subsequent bone cuts. A major problem is in the instrumentation to indicate the location of the femoral cuts which relies upon the "experience" or "eyeballing" of the surgeon. Over the years, two basic instrument system designs have become popular.
In one design (anterior referencing), the total knee alignment system takes its point of reference from a centrally placed rod and careful attention is given to the patellofemoral joint by using an anteriorly placed feeler gage. The distal femoral cut is consistent with the thickness of the prosthesis.
This instrument system operates on the principle of anatomic anterior and distal femoral cuts to allow proper ligament balancing and stability in extension as well as consistent patellofemoral placement on the anterior surface. The femur is not notched, and the anterior surface of the femoral prosthesis not elevated above the anterior surface of the femur. Notching the femur may cause a decrease in strength of the distal femur. Elevation of the anterior surface of the prosthesis will cause extremely high patellofemoral pressures in a joint that seems to be prone to a high rate of post-operative failure.
By establishing the anterior femoral cut as the benchmark or datum starting point, however, the anterior referencing instruments result in the installation of a knee prosthesis which sacrifices consistent stability in flexion due to the formation of a posterior femoral condylar cut that may leave the posterior space either too wide or too narrow. This can cause instability in flexion, or restrict flexion and cause increased wear.
The second type of instrument design (posterior referencing) is based on the concept that the flexion and extension stability are more important and the patellofemoral joint is of secondary importance. This system also uses an intramedullary rod for referencing. Although I consider all three joints as "important", when a compromise must be made, the posterior referencing systems compromise the patellofemoral joint while the anterior reference systems sacrifice stability in flexion (the posterior tibial femoral joint). Both systems allegedly equally address the distal tibial-femoral space. Neither consistently addresses the distal rotation of the femoral component.